Moderate ductility of the bracing joints with preloaded bolts

Abstract

In Europe, concentrically braced frames (CBFs) with double angle bracings are the most frequent lateral-resistant structural systems. Although their standard bolted connections provide economic and robust solutions for the static loading, they are assumed to have almost zero ductility under strong earthquake actions. In order to avoid a brittle failure, the current seismic design requirements of European Code provisions require these joints to have sufficient over-strength, and to remain elastic for the design earthquake. While this is a safe approach for the high seismicity situations, it causes costly solutions for the buildings designed in the low-to-moderate seismicity context. Therefore, mainly for economy reasons, design engineers usually choose standard non-seismic approach (DCL) for the design of CBFs located in low-to-moderate seismic regions. However, such a choice may lead to unsafe solutions, since no effort is paid to control ductility. To combine safety and economy in this context, a new specific method has been studied in the recently concluded EU-RFCS MEAKADO project. To explore the inherent ductility provided by standard double-angle bracing joints with preloaded bolts and respecting new edge-spacing requirements, full scale tests have been performed as a project task. This article analyses the test results, and quantifies the ductility provided by the bolt hole ovalization and the slippage of preloaded bolts of the bracing joints not fulfilling the current over-strength design criteria. The test data has been analysed by means of LVDTs, strain gauges and thermal images. Such ductility and dissipation resources are traditionally not desired from a high seismicity design point of view, but may satisfy the low horizontal shear demand of the buildings designed for the low-to-moderate earthquake zones.